Cast coating with improved speed and quality

ABSTRACT

CAST COATING IS CARRIED OUT UPON A PAPER SUBSTRATE HAVING A SURFACE OF IMPROVED LEVELNESS AND UNIFORMITY AND PREFERABLY SURFACES WITH A WATER INSOLUBLE COATING WHICH IS DESIRABLY FINISHED TO MAXIMIZE FLATNESS AND GLOSS, IN ORDER TO SUBSTANTIALLY INCREASE CAST COATING SPEED, DECREASE CAST COATING COMSUMPTION AND, IN MANY CIRCUMSTANCES, IMPROVE CAST COAT QUALITY.

P. o. HAIN 3,829, CAST COATING WITH IMPROVED SPEED AND QUALITY Aug. 13, 1974 Filed Feb. 25, 1972 United States Patent 3,829,325 CAST COATING WITH IMPROVED SPEED AND QUALITY Paul O. Hain, Hamilton, Ohio, assignor to Champion International Corporation Filed Feb. 23, 1972, Ser. No. 228,588 Int. Cl. B44d 1/44; D21h 1/22 U.S. Cl. 117-64 C 13 Claims ABSTRACT OF THE DISCLOSURE Cast coating constitutes a well known process for finishing paper to provide a surface having high gloss and specular appearance. Unfortunately, cast coating requires a heavy deposit of coating material, usually at least 18 pounds of coating per ream, and the process is slow since water must be substantially completely removed from the coating through the paper to enable the coating to separate cleanly from the casting drum. Also, many grades of paper cannot be cast coated unless even larger amounts of coating are employed, thus rendering the process even more costly and less rapid. Moreover, there are limitations in the amount of coating which can be applied at the nip. Still further, the thicker the required coating, the less satisfactory and practicable is the product since the thicker coatings break and crack, limiting the end use capabilities of the coated paper.

Prior efforts to reduce the required coating wegiht, to increase speed and improve coating quality, have not been fully successful. Thus, precoating has been used, with limited success, in order to reduce the surface volume of the paper and, thereby, reduce the amount of coating needed during the casting operation. In conventional practice, the paper is coated with about 6 pounds of coating on the paper machine and then cast coated as taught in United States Pat. No. 3,044,896 applying a first wet coat weighing about 6 pounds, and then applying an additional 6 pounds of coating at the nip. The result is a modest improvement in cast coating speed, but no reduction in coating weight. Also, it is possible with three coatings, as indicated, to handle paper which could not be coated satisfactorily with a single coating.

supercalendering of the base sheet was tried many years ago, but the anticipated improvement in speed and quality was not obtained. It was believed that this failure was based on the densification caused by supercalendering slowing the escape of water vapor through the sheet while it was on the casting drum.

It has now been found that it is the nonuniformity in the thickness distribution of the applied wet coating which controls both product quality and casting speed. The coating weight is actually an average of a large number of microareas. For example, if we obtain a cast coating weight of 12 lb. per ream, there may be microareas with twice that weight and some with half. The magnitude and effect of these dilferences is reflected in the area, depth and distribution of the prominences and depressions of the base stock. Examining the failure of supercalendering to improve casting speed on this basis and on the strength of our present experience it seems likely that densification had little or no effect on the outcome. Rather, the inability of ordinary supercalendering to effect a substantial enough change in surface flatness must be the real reason.

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What appears to occur is that the portions of the coating which are thickest dry more slowly and drum speed must be limited by the drying of the heaviest microareas. If drying is incomplete, the paper will stick to the drum. The heavy microareas also limit drum temperature. These heavily coated microareas also release water more slowly through the coating layer and paper. If attempts are made to force casting at higher speeds by increasing drum temperature, water in these heavily coated microareas will not escape as readily and will boil at the drum surface. This localized boiling results in a porous, foamy structure which will not accept ink properly or satisfactorily reproduce half-tone structure.

Even when the thicker areas are adequately handled, the thinner areas are overheated which builds stresses and strains into the product which again leads to an unsatisfactory appearance.

Whereas prior experience indicates that cast coating speed and performance are determined by the ease with which the water in the coating can be removed through the paper substrate, I have found that the paper substrate should have an improved uniformity of cellulose microstructure near the surface thereof and the paper is preferably surfaced with a water insoluble microporous coating. Furthermore, it is advantageous to finish the precoated base to the highest flatness and gloss despite the reduction in pore structure which occurs and the previous adverse experience using supercalendered stock. This new result is believed to follow from the fact that the coating of more uniform thickness can more uniformly release its water, this water being easily removed as a vapor through a microporous coating despite the decrease in pore structure of the paper surface and the high gloss of the coating. These results can be visually documented by cast coating atop a base coat which contains a fluorescent dye. If the base coat does not have the required flatness, then the cast coated final product, under ultraviolet light, reveals a pattern establishing the nonuniformity in the thickness of the cast coat. The size, distribution and contrast of the pattern correlate with the quality of the final product, and the speed with which cast coat-- ing can be performed.

It is desired to stress that this invention, unlike the art, actually enables a reduction in overall material consumption. Thus, the art might eifectively use either 18 pounds of coating applied in one operation, or a 6 pound precoat and two 6 pound coatings in the casting operation, totaling the same 18 pounds, but applied in three layers. Regardless of the method of application, the same total coating weight is needed in practice. In contrast, an uncoated base stock can be prepared with 7 pounds of base coating which is selectively increased in flatness and glossed as taught in my prior United States Pats. 3,268,- 354; 3,338,735; and 3,338,736, and then cast coated using 7 pounds or less of coating applied at the nip to produce a cast coated product which is fully equivalent to commercial quality, but which has only 14 pounds of total coating weight (or less) applied thereto. The saving in material costs is self evident, and the thinner coating is less brittle, providing a distinct advantage. Also, the cast coating speed is greatly increased, in actual practice approximately doubled and, in some instances, approximately tripled.

Referring more particularly to the paper substrate on which the cast coating is applied, these are preferably provided as taught in my prior United States Pat. 3,268,- 354 in which a dry fibrous cellulosic substrate has applied thereto an aqueous coating containing finely divided solid mineral filler, usually in major proportion, in admixture with a minor weight proportion of a non-water absorptive, film-forming, heat-softenable resin in particulate form. The coated substrate is subjected to an elevated temperature applied from the coated side of the paper in order to rapidly dry the aqueous coating until it is dry to the touch and thereby form a pressure deformable layer at the surface of the paper substrate adjacent the coating. When this coated substrate with its water content providing the pressure deformable layer as noted is passed between a pair of turning rolls, at least one of which is provided with a heated finishing surface to cause the resin particles to at least partly coalesce, there is produced a coated substrate with a finished print-receptive surface of relatively high gloss and in which the surface of the paper is remarkably level. While the coating is visually continuous, and is preferably constituted by non-water absorptive resin particles bonding the particles of filler into a film, the film is microporous so that, while a drop of water will not quickly penetrate the film, water in vapor form can easily pass through the film.

While the non-water absorptive, fihn-forming, heat-softenable resin particles and the proportion of filler described in my prior Pat. 3,268,354 may be used, it is also possible to employ a larger proportion of resin as in my prior Pat. 3,338,736, and it is particularly preferred to employ a resin latex with from 70-90% of mineral filler as in my prior Pat. 3,338,735.

Regardless of the specific nature of the resin which is employed, the aqueous coating containing the same is deposited on the paper substrate in relatively small amount, preferably less than about 9 pounds of dry coating per ream (3300 square feet) and more preferably in an amount of 4-7 pounds per ream. Usually, at least about 1 pound, preferably at least 3 pounds per ream are required to carry out the process.

As described in my prior patents, the finishing surface of the turning rolls is desirably heated to a temperature within the range of 190-350 F. using a nip pressure of at least about 500 p.s.i., preferably about 600 to 7500 p.s.i. The rapid heating is preferably effected using radiant heat or gas burners in close proximity to the paper so that the desired drying of the aqueous coating is obtained in up to 1.5 seconds, preferably less than one second. While the desired extent of drying can be roughly approximated by noting that the surface has become dry to the touch, it is preferred to note the extent of drying more precisely using an infrared pyrometer which is held within about inch of the heated paper surface as more particularly described in my Pat. 3,338,736, with rapid heating being employed until the exposed surface of the coated paper has a surface temperature in the range of from about 160 F. to about 195 F. With operation being carried out with greater precision, as indicated, it becomes possible to employ a larger proportion of resin in the aqueous coating, and, while it is possible to proceed in this manner in the present invention, it is nonetheless preferred to employ a major weight proportion of mineral filler. Also, and again using greater precision and control in the operating procedure, it becomes feasible to employ resins which are at least partially water soluble and also rubbery latex particles.

The specific nature of the resin particles which are employed in this invention is of secondary consideration and the invention will be illustrated utilizing a styrene-butadiene latex.

The styrene-butadiene latices for paper coating are conventional and normally contain about 45-75% styrene, balance butadiene. These may be modified for improved characteristics with small amounts of comonomers such as unsaturated acids (acrylic or methacrylic acid) to provide an acid number of from 5-50.

From the standpoint of the composition which is employed for the cast coating operation, any composition which can be employed in conventional cast coating can be employed in the present invention, though this invention permits the viscosity of the coating to be somewhat reduced in order to facilitate the application of a smaller coating weight of the cast coating material.

The invention is illustrated by the following example of presently preferred operation.

EXAMPLE The invention is illustrated using a standard uncoated grade of paper known as duplicator stock. It has a basis weight of 47 pounds and a Shefiield smoothness of on the felt side. The fiber content of the paper is about 60% hardwood and 40% long fiber pine.

The base stock described above is first coated by applying thereto 6 pounds per ream (dry weight) of an aqueous coating having a solids content of 57% by weight and containing, by weight, 85 parts of #1 coating clay, 15 parts calcium carbonate, 12 parts of styrene-butadiene latex in which the copolymer is formed by aqueous emulsion copolymerization of 65 parts styrene, 35 parts butadiene, and 2 parts methacrylic acid, and /2 part high melting point wax emulsion. This aqueous coating is applied to the felt side of the paper and then dried to a surface temperature of 170 F. in about one second using radiant heat applied to the coated side. The partially dried wet coated paper is then passed through 4 nip-s, with the pressure at the nip being 800 p.s.i. utilizing two Badall finishing rolls maintained at a temperature of 300 F. The paper is treated at a speed of 700 feet per minute to provide a gloss of about 60. Commercially available styrenebutadiene latices which may be used to provide equivalent results are Dow 650 and Dow 620 latices.

The coated paper so-provided is significantly leveled by the treatment, and the coating on the finished paper is visually continuous and microporous.

The above coated paper is then used to provide a base for cast coating, the cast coating composition being applied to the leveled, coated side of the paper. Approximately 7 pounds per ream of an aqueous cast coating composition are applied, the cast coating composition being as follows:

Parts by weight Calcium carbonate 85 #1 coating clay 15 Ammonium casein 9 Styrene-butadiene latex (same as in precoat) 8 Oil containing 81% oleic acid The above coating composition is applied at 54% solids content which is somewhat lower than the usual solids content of 57-58%, in order to obtain the desired lower than usual coat weight on the smooth raw stock.

The paper is cast dry back using a drum temperature of 188 F. and a speed of 350 feet per minute. The cast coating arrangement is shown in the drawing in which the cast coating drum is identified at 10, the drum being rotated as shown by the arrow. The coated paper 11, with its previously coated side positioned on the undersurface thereof, is advanced from a supply, not shown, past a. coating apparatus 12 where the precoated surface receives a wet coating of the composition described above from a rotating applicator roll 13 revolving in a coating pan 14. The paper 11, with the cast coat material applied as indlcated, is then advanced past a guide roll 15 to a press roll 16 which squeezes off the excess coating into a pan 17. In this example, press roll 16 has a diameter of 3 inches and a -P & J Plastometer hardness of 25 and forms a n p which is 0.2 inch wide. The press roll is forced against the casting roll 10 with a pressure of pounds per lineal inch of nip (650 p.s.i.). This is not conventional operation since the nip is usually wider, e.g., at least 0.5 inch in length and the press roll is normally formed of somewhat softer material. In this invention, it is preferred that the press roll be at least about the hardness specified, or harder, with the nip being preferably not wider than 0.4 Inch. The nip should be as narrow as possible consistent with avoiding damage of the drum, desirably about 0.1 Inch in width. Also, in this example, the casting drum 10 has a diameter of 12 feet.

The cast coated product in this example has a gloss of 85 and the apparent gloss of the coating is even higher because of improved flatness. On the other hand, the speed of operation in this example is twice as fast as is customarily employed.

Commercially available styrene-butadiene latices which may be used to provide equivalent results are Dow 650 and Dow 620 latices.

As will be evident, the water which is removed from the coating in order to dry the same must pass through the microporous precoat and the reduced porosity of the cellulose top layer associated therewith. With the higher speed of operation contemplated herein, it has been found that removal of water is particularly benefitted by the blowing of air upon the back of the paper as indicated by nozzles 18.

Also, and if desired, water can be applied as indicated by arrow 19 or arrow 20 to initially reduce the drying rate when the wet coating is pressed against the casting drum 10. Water applied as indicated at 19 serves to lower the drum temperature just before it is contacted by the wet coated paper 11, and this is helpful to enable the utilization of a casting drum operated at higher overall temperature.

The casting drum may be heated to a temperature of from 150 F. or higher so long as the temperature of the drum is not so high as to cause boiling of the water in the aqueous coating. The highest drum temperature is approximately 210 F., but so long as disruptive boiling at the coated surface is avoided, this upper limit of temperature is not a precise figure.

It will also be apparent that the speed of operation is limited by the requirement that sufiicient water he removed to enable the dried coating to separate cleanly from the finishing surface of the casting drum. Accordingly, and in this invention, if I use a harder press roll so as to form a narrower nip, then the cast coatings can be formed with coating compositions having a lesser proportion of water. With less water to remove, still faster operation is possible.

The invention is defined in the claims which follow.

I claim:

1. A method of cast coating comprising depositing an aqueous cast coat composition onto the coated side of a paper substrate precoated with from l-9 pounds per ream of a visually continuous dry microporous coating with the surface of the paper underlying the precoat being selectively treated to level and improve the uniformity of the surface of the precoated paper, and thereby improve the uniformity in the thickness distribution of the applied coating on the microscopic scale, passing the precoated paper with the wet cast coat composition theeron into a nip between a press roll and a casting drum with the wet coating against the casting roll to squeeze out excess coating, said casting roll being heated to a temperature of at least about 150 F. to heat the water in the cast coat composition and cause it to vaporize and pass through the precoat for removal through the paper until the cast coat composition has dried, and then removing the cast coat paper so-produced.

2. A method as recited in claim 1 in which said paper substrate is precoated with from 4-7 pounds per ream of said microporous coating.

3. A method as recited in claim 1 in which said precoat comprises a major weight proportion of mineral filler bonded into a film by a minor weight proportion of at least partially coalesced particles of non-water absorptive, film-forming, heat-softenable resin.

4. A method as recited in claim 1 in which said precoat comprises from 70-97% by weight of mineral filler adhesively secured by from 330% by weight of rubbery latex particles bonding the particles of said filler into a film.

5. A method as recited in claim 4 in which said latex is a styrene-butadiene latex.

6. A method as recited in claim 1 in which the total weight of coating is smaller than is normally needed for the cast coating of the selected paper.

7. A method as recited in claim 1 in which the cast coating speed is higher than can normally be employed for the cast coating of the selected paper.

8. A method as recited in claim 1 in which the press roll has a hardness on the P & J Plastometer of at least about 25.

9. A method as recited in claim 1 in which the press roll establishes a nip with the casting drum having a width of 0.4 inch or less.

10. A method as recited in claim 9 in which the press roll establishes a nip with the casting drum of about 0.2 inch.

11. A method as recited in claim 1 in which air is blown on the rear of the partially dried paper to speed the drying operation.

12. A method of cast coating comprising precoating a paper substrate with from 1-9 pounds per ream of a visually continuous microporous coating by applying to said substrate an aqueous coating containing a major weight proportion of finely divided solid mineral filler in admixture with a minor weight proportion of non-water absorptive, film-forming, heat-softenable resin in particulate form, subjecting the wet-coated substrate to an elevated temperature from the coated side of the paper to rapidly heat the exposed surface to a surface temperature in the range of from about 160 F. to about 195 F. and provide a pressure deformable layer at the surface of the paper substrate adjacent the coating, and then passing the partially dried coated paper so-produced through the nip between a pair of turning rolls, at least one of which is provided with a heated finishing surface, to cause the resin particles to at least partly coalesce to provide a glossed surface in which the surface of the paper is leveled and rendered uniform, depositing an aqueous cast coat composition onto the precoated surface, passing the precoated paper with the wet cast coat composition thereon into a nip between a press roll and a casting drum with the wet coating against the casting roll to squeeze out excess coating, said casting roll being heated to a temperature of at least about F. to heat the water in the cast coat composition and cause it to vaporize and pass through the precoat for removal through the paper until the cast coat composition has dried, and then removing the cast coat paper so-produced.

13. A method as recited in claim 12 in which at least about 14 pounds of total coating is applied.

References Cited UNITED STATES PATENTS 3,268,354 8/1966 Hain 1l7-65.2 3,044,896 7/1962 Warner 117-64 C 3,300,364 1/1967 Gottwald et al. 117-64 C 2,919,205 12/ 1959 Hart 117-64 C 3,110,612 11/1963 Gottwald et a1 117-64 C 3,115,438 12/1963 Randall 117-64 R 2,214,565 9/1940 Montgomery et al. 117-64 C 3,028,258 4/1962 Rice 117-64 C 3,268,358 8/1966 Hain 117-64 C 3,338,735 8/1967 Hain 117-64 R 3,338,736 8/1967 Hain 117-64 R 3,296,014 1/ 1967 Williams 117-64 C 2,955,970 10/1960 Rice 117-64 R 3,281,267 10/1966 Rice 117-64 R WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner U.S. Cl. X.R. 

